Office seats and chairs of the type equipped with mechanisms of adjustment and adaptation so as to permit a wide range of users to sit comfortably are certainly well-known. In contrast to lower-priced models of the type with a seating position and seat back which are fixed, they incorporate, in addition to a device for adjusting the height of the seat, the use of a device for the controlled swinging of the seat back, generally located in the part immediately beneath the seat plane and integral to it. This device is primarily activated via a protruding lever that can easily be gripped and therefore rotated in one direction or another, until the internal mechanism releases the articulation.
In short, it is therefore possible to maintain that the following are known:                1. chairs in which the seat and seat back are designed with separate body shells, which are connected in such a way that an inclination of the seat back corresponds to a parallel downward movement of the seat plane;        2. chairs in which only the seat back is freely swinging;        3. chairs in which the raising of the seat plane corresponds to the inclination of the seat back;        4. chairs in which both the seat and the seat back are individually adjustable;        5. finally, seats in which both the seat plane and the seat back, which are interconnected, perform a synchronized inclination movement.        
Almost all of the solutions mentioned involve, for the purposes of cushioning the swinging movement of the seat back, the seat or both during the displacements due to the movements of the user, a means of cushioning, usually of the type consisting of a compression spring.
Prior Art
In the field of dynamic seating devices, a first dynamic device for chairs is described in FR2075176 (Suspa), involving a base equipped with a number of support arms, from which an upright rises, composed of a gas cylinder. The end of said gas cylinder, from which the activation button of the piston protrudes, is inserted inside a hole in the shape of a truncated cone, extracted monolithically from the containing box structure of the device.
More significant is U.S. Pat. No. 4,986,601 (Inoue), which mentions the swinging mechanism to support a seat and the seat back of a chair, consisting of a central column that extends from the base of the chair, a first support clamp connected to the central column by means of a pedestal, a second support clamp connected to the chair and hinged to the first support clamp by means of a shaft, a third support clamp connected to the rear support and hinged by means of a shaft to the second support clamp, and a shaft that hinges the first support clamp to the third support clamp. U.S. Pat. No. 4,986,601 (Inoue) also provides for a gas piston with a valve that can be actuated by a rod in the central column to selectively adjust the height of the column. Finally, U.S. Pat. No. 4,986,601 (Inoue) also provides for spring mechanisms to exert an opposing force on the second support clamp, positioned between the first and second clamps.
DE0198056 (Neumuller) is of interest. This is a swinging mechanism to support the seat plane and seat back of a chair, consisting of a central column that extends from the base of the chair, a first support clamp connected to the central column, a second support clamp connected to the seat plane and pivoted to the first clamp by a first pivot, a third support clamp connected to the seat back and hinged to the second support clamp by a second pivot, in addition to a mechanism that pivots the first support clamp to the third support clamp. DE0198056 (Neumuller) also provides for spring mechanisms positioned between the first and second support clamps, to exert an opposing force to the movement of the second clamp. DE0198056 (Neumuller) also provides for locking mechanisms, consisting of a third pivot connected to the first clamp and a fourth pivot connected to the second clamp, working together with a number of disks equipped with an elongated slot through which the pivot passes. A rod associated with the respective pivot provides the friction anchoring of the disks, which are thus held together.
Prior Art Concerning the Invention
The specific function for which provision is made for the presence of a spring in the field of chair structures, particularly chairs of the office type, which are equipped with appropriate articulations between the seat and seat back components, is principally the need to provide a means of counteracting the action exerted by the shifting of the user's weight during the swinging that occurs in the act of sitting down. Generally, it happens that the tension loads for which said springs need to be calibrated are considerable, which means that the consequent adjustment manoeuver involving the pre-loading of the same spring, for the purpose of allowing a greater or lesser counteraction in relation to the weight and requirements of the user, presents aspects of difficulty. Such aspects of difficulty principally concern both the structure of the pre-loading mechanism and the accessibility of the same adjustment device on the part of the user.
In the prior art, adjustment devices for the pre-loading of the spring in articulation structures for chairs are known, particularly for office chairs. In most cases, said pre-loading mechanisms primarily use devices composed of bevel gears, or via the use of threaded means are capable of guaranteeing the gearing-down necessary to permit the user to adjust the pre-loading of the spring by reducing the effort required to achieve it.
WO 91/14385 (Hansen) presents a description of a seating mechanism in which the seat plane is of the elastically inclinable type. A base support column constitutes the support for a clamp positioned obliquely to one end joined near the top of said support column, while, opposite to said clamp, is a hinged metal component for the support of a seat plane at right angles to a laminated leaf spring interposed between said clamp and said component for the support of the seat plane. Said spring is equipped with a pre-tensioning device composed of a threaded guide which, at one end, holds said spring in an area close to the fulcrum area, while with the opposite end it crosses said clamp so as to be able to be held by a threaded knob. The manipulation of this threaded knob by the user permits the discretionary variation of the pre-loading of the spring.
U.S. Pat. No. 4,711,491 (Ginat) contains a description of a swivelling and folding mechanism for seats. Within the limits of the cushioning device, U.S. Pat. No. 4,711,491 (Ginat) describes the latter as being composed of a helical spring which is interposed between a hollow cylindrical housing inside a clamp projecting from the upper end of a support column, and a component for the support of a seat plane pivoted to said clamp, near to the opposite end of which the latter is anchored to the same support column. A housing for holding the corresponding end of said helical spring is located within said seating component. Said hollow cylindrical housing is centrally equipped with a threaded seat for the passage of a threaded pin, into which is cut, near to the part which will be placed inside said cylindrical housing, a retaining body for the corresponding end of the spring, while at the opposite end, said threaded pin is equipped with a knob.
U.S. Pat. No. 5,207,479 (Wickman et al.) contains the description of a control mechanism for the swinging motions of a seat of the type equipped with an articulation for the movement of the seating plane and/or the seat back. The information contained in said patent document includes a pre-loading mechanism of a spring which includes an adjustable threaded means and a tension arm equipped with a mechanical linkage. Said mechanical linkage, on which the threaded means is positioned via the rotation of the latter, enables the variation of the pre-loading of the spring and the connected damper, which are located between the end of said mechanical linkage, opposite to the end at which said threaded means is positioned. In addition, said mechanism is equipped with means for determining the maximum and minimum extension of the swinging movement of the seat plane.
Disadvantages
All of the abovementioned solutions have certain limitations. In short, in the opinion of the applicant, all of the cited solutions have the disadvantage of requiring the user to make considerable use of the activation of the pre-loading mechanism. This use principally involves the need, given the considerable gearing-down necessary to the purpose, for repeated maneuvers to activate the pre-loading adjustment mechanism.
Likewise, all of the previously described solutions have the disadvantage of having said device for the adjustment of the pre-loading of the spring positioned in front of and underneath the seat plane. This positioning forces the user into an unnatural contortion at the time when the user decides to perform a recalibration of the pre-loading of the spring.
As far as the solution described in WO91/14385 (Hansen) is more specifically concerned, the principal limitation consists in having constructed said pre-loading device specifically for use in chair structures that utilize springs of the laminated type. Another limitation, in the opinion of the applicant, appears to be that said pre-loading device can only be adjusted with ease if the spring is in its position of highest compression.
Both the solution described in U.S. Pat. No. 4,711,491 (Ginat) and the solution described in U.S. Pat. No. 5,207,479 (Wickman et al.) possess a relevant disadvantageous feature in the fact that the device for adjusting the pre-loading of the spring is positioned underneath the seat plane near to the front part, with the consequence of limiting its accessibility to the user. In addition, such a solution is intended specifically for structures of seats in which the seat plane is articulated in only one place. A further limitation can be found in the fact that, in said solution, it seems that an adjustment of the pre-loading of the spring can only be performed with ease when the user is not sitting down on the seat. Indeed, if the user is seated, the combination of the thrust of the spring and the weight of the user prove to be circumstances that are capable of increasing wear and tear, in such a way as to make said adjustment noticeably difficult.
As regards the solution described in U.S. Pat. No. 5,207,479 (Wickman et al.), apart from the indicated limitation of making it difficult to access the adjustment of the spring, by positioning said control underneath and towards the front of the seat plane, in the opinion of the applicant, the latter limitation consists of the notable construction-related complication that the interposition of said mechanical linkage involves for the required specific structure of any articulation of the seat. This specific structure reflects negatively on the same industrialization process by resulting in an increase in the time and costs required to realize it and furthermore, in the opinion of the applicant, involving the reduced functionality of the whole.